The membranes of lipid droplets in cells are lamellar structures, containing amphilic lipids (fatty acids and partial glycerides), and are continuous with membrane leaflets of smooth endoplasmic reticulum and Golgi. In mammary glands, a mechanism of milk lipid droplet formation is proposed based on electron microscope and freeze-fracture studies. In developing fat cells membrane leaflets are sites for esterification of fatty acids and storage of triacylgycerol. In hepatic cells lamellae of lipid droplets are sites of intracellular lipolysis. Electron microscope autoradiography demonstrated the route of chylomicron lipids across the capillary wall to cells of lactating mammary gland. With monolayer technics we studied the action of purified lipoprotein lipase on trioleoylglycerol. Products of hydrolysis moved laterially in a lipid-water interface and both movement and lipolysis are enhanced when the products are removed from the interface at a distal point. The findings support the model proposed for transport of lipids by lateral diffusion in cell membranes. BIBLIOGRAPHIC REFERENCES: Mendelson, C.R., Zinder, O., Blanchette-Mackie, E.J., Chernick, S.S. and Scow, R.O.: Lipoprotein lipase and lipid metabolism in mammary gland. J. Dairy Science 60: 666-676, 1977. Scow, R.O., Chernick, S.S. and Fleck, T.R.: Lipoprotein lipase and uptake of triacylglycerol, cholesterol and phosphatidylcholine from chylomicrons by mammary and adipose tissue of lactating rats in vivo. Biochim. Biophys. Acta 487: 297-306, 1977.